Flexible thread systems for high-pressure vessels and the like

ABSTRACT

A flexible thread system for use on screw-type closure structures for high-pressure vessels and the like wherein a gullet, with a paraboliclike taper, is provided below the normal root portion of one, or both, of the mating male and female threads of the screw-type closure, the gullet or gullets enabling the thread cross section to deflect as a lever in order to enable the closure threads to be uniformly stressed and to accommodate the pressurized stress patterns of the vessel system.

n 1 sense United States Patent 56] References Cited UNITED STATESPATENTS [72] Inventor PeterF.Rossmann Grosse Pointe Farms, Mich.

1 mm WWW mT.m "AwO m nL fl m mmae mEwW h P Gmm 0 .8. 4 5 s W N /nfl 1 al Umh M w a flo 3 60m 6 0M0 nn 2 PA 0 N ho 0 SN t .mm ta m H PW Po an mM8 WW C m4 0. mum in WwF m L m mmum -n v m n fiomanomfl SNJNIC73 0 e Nee n dmm W ms AHPA lllil 253 224 [ill ABSTRACT: A flexible thread systemfor use on screw cl osure structures for high-pressure vessels and thelike wherein a gullet, with a paraboliclike taper, is provided below thenormal root portion of one, or both, of the mating male and femalethreads of the screw-type closure, the gullet or gullets enabling thethread cross section to deflect as a lever in 220/39 R order to enablethe closure threads to be uniformly stressed B651! 39/08 and toaccommodate the pressurized stress patterns of the was 220/39 R, 3 selsystem.

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FLEXIBLE THREAD SYSTEMS FOREIGN-PRESSURE VESSELS AND THE LIKECROSS-REFERENCE TO RELATED APPLICATION This application is acontinuationin-part of my prior copending application Ser. No. 710,205filed Mar. 4, 1968, now U.S. Pat. No. 3,487,442.

BACKGROUND OF THE INVENTION This invention relates to an improvement inclosure means of the threaded type for metallic vessels utilized forhigh-pressure applications and more particularly to an improved threadsystem which has an inherent flexible characteristic.

In certain high-pressure applications, such as for example, thoseinvolving isostatic vessels which are provided with closure means of thethreaded type, it is desirable that the closure threads be uniformlystressed or that they accommodate the pressurized stress patterns of thevessel system.

In pressure vessel systems using threaded closures, which are thesimplest and preferred constructions, the stress distribution on thethreads has a paraboliclike function with the highest axial stressesimposed on the threads nearest the internal pressure in the vessel andwith the stresses decreasing in the threads outward from the innerportion of the closure. There are also radial stresses in the pressurevessel or nut element which further distort or increase the stresses inthe threads. Accordingly, the closure constructions in high-pressurevessels have necessarily been required to have overdesigned crosssections inorder to provide adequate safety factors, since not all ofthe closure threads are equally and efficiently stressed to their loadcarrying capacity.

The invention may be applied to isostatic pressure vessels or topressure vessels of other types, or to pressure vessel systems whichrequire parallel or helical screw threads in the closure constructions.The closure elements may be either of the continuous thread screw andnut type, or have interrupted parallel or helical threads such as are incommon use for the breeches of artillery guns. In all such cases, a truethread profile adapted to the particular application may be used.

The principal object of the invention has been the provision of a noveland improved thread system.

More particularly, it has been an object of the invention to provide athread system for use with pressure vessel closures and in which theclosure threads are either uniformly stressed or otherwise accommodatethe pressurized stress patterns of the vessel system.

Another object of the invention has been the provision of a threadsystem having a flexibility characteristic which enables the threadcross section to deflect as a lever.

Still another object of the invention has been the provision of such athread system in which the complex stresses in a pressure vessel closuresystem can be accommodated without the necessity for overdesign in crosssection strength and without the necessity for interposing anindependent stress-absorbing element between the threads.

Other and further objects, features and advantages of the invention willbe apparent from the following description of the invention.

SUMMARY OF THE INVENTION In accordance with the invention, there isprovided a thread system for use on screw-type closure structures forhigh-pressure vessels and the like wherein threaded male and femalemembers are in threaded engagement. Typically a threadedtype plugclosure member is screwed into a complementary threaded opening into apressure vessel. The thread system involves the provision of a gullet orgroove below the normal root portion of the male thread, the femalethread or both such threads. The gullet or groove, the depth of whichpreferably has a paraboliclike taper, establishes a flexibilitycharacteristic enabling the thread cross section to deflect as a leverin order to enable the closure threads to be stressed uniformly or toaccommodate pressurized stress patterns of the vessel system. The gulletdepth in the preferred case of a paraboliclike tapered gullet, isprogrammed to correspond to the stress distribution on the threads sothat all portions of the threads are deflected by amounts which resultin substantially uniform unit stresses in the metal. The terms taper andtapered are intended to mean axially profiled.

BRIEF Description OF THE DRAWINGS The invention will now be described ingreater detail with reference to the appended drawings in which:

FIG. I is a view in central vertical section of the closure area of ahigh-pressure vessel;

FIG. la is a diagrammatic view in central vertical section of a portionof the pressure vessel thread of FIG. ll;

FIG. 2 shows a portion of the threaded closure of FIG. I drawn to anenlarged scale for purposes of clarity;

FIG. 2a is a view similar to FIG. 2 but illustrating, in dashed linesand to an exaggerated extent, thread deflections;

FIG. 3 is a view similar to FIG. 2 but showing a modified constructionfor the gullets associated with the threads; and

FIG. 3a is a view similar to FIG. 3, but illustrating a further modifiedconstruction and also, in dashed lines and to an exaggerated extent,thread deflections.

With reference now to FIGS. 1 and 2 in particular, the highpressurevessel 10 may have any configuration and only the portion thereofprovided with a threaded opening 111 into which a threaded plug 12 isscrewed to close off the interior of the vessel has been illustrated.The female thread provided in the opening 11 is designated 13 while themale thread provided on the closure 12 is designated 14.

Buttress-type threads are illustrated for the screwtype closure but theimproved thread construction is not limited to this particular type ofthread, as has been previously indicated.

The pitch diameter of the thread system is indicated by the dimensionline D]. The normal root diameter of the female thread 13 is representedby dimension line D2, and the root diameter of the male thread 14 isrepresented by dimension line D3. Associated with the normal rootportion of the female thread 13 is a helical groove or gullet 15, andsimilarly associated with the root portion of the male thread 14 is ahelical groove or gullet 16.

The stress distribution on the threads has a paraboliclike function,with the greatest stress being on the first load bearing threads 5 and 6of the vessel wall and closure plug, respectively. The axial profile ofthe gullet depth is made to correspond to the stress distributionfunction, as described below, in order to uniformly stress the threads.Thus the depth of the helical gullet I5 is not uniform throughout theaxial length of female thread 13 but rather progressively increases inthe direction of the end of the thread nearest the interior of thevessel 10. As seen in the diametral section of FIG. I, the root ofgullet 15 has a paraboliclike taper represented by line Ll-Ll.

Similarly, the depth of helical gullet 16 is not uniform throughout theaxial length of thread 14 but rather it also progressively increases inthe direction of the end of the thread nearest the interior of vessel10. As seen in FIG. 1, the root of the gullet 16 has a parabolicliketaper represented by line L2L2.

The effect of the gullets is to reduce the stiffness of the surroundingmetal bodies and hence facilitate bending of the metal under stress.Thus the provision of the gullet at the base of the female threadpermits the female thread wall to bend as required to accommodate theforce exerted on it by the mating male thread, and vice versa. The depthof the gullets and the extent and direction of the taper which changesthis depth from point to point are selected so that all portions of thethreads are deflected by amounts which result in substantially uniformstresses in the metal.

Referring now to FIG. la, there is diagrammatically illustrated theaxial profile of the vessel thread gullet along with certain dimensionsused in calculating the gullet depth. An inverted corresponding figurewould represent the axial profile of the gullet depth of the closureplug thread. The dimension b is the distance along the threads measuredfrom the underside of the first load bearing thread 5. The dimension dis the gullet depth at the distance b measured from the thread tip orthe innermost diameter D3 of the vessel 10. In the case of the malethreads the distance d would be measured from the diameter D2. Thedimension D is the standard depth of the thread without the gullet,i.e., it is one-half the difference of the normal root diameter D2 ofthe female thread 13 and the normal root diameter D3 of the'male thread14. The dimension B is the axial length of the threaded portion of thevessel 10.

The stress distribution function of both the vessel and closure plugthreads is either exponential or paraboliclike, de-

where:

X the distance along the threads measured from the underside of thefirst load-bearing thread and I= the quotient of the load on aparticular thread divided by the average load on all the threads.

By making the axial profile of the gullet depths correspond to the loaddistribution function, the threads will deflect in such a manner as tobe uniformly stressed. Thus any point on the curve Ll-Ll forming theprofile of the thread gullet depths is given by substituting thefollowing dimensions into the stress distribution function:

At a distance b from the underside of the first load-bearing thread thegullet will have a depth d which may be determined by solving theexpression:

6.31 bd/BD 0.75 bd /BD 0.0156 rfl/D l It is not necessary that both themale and female threaded members be provided with gullets as long as oneor the other is provided with a gullet. In embodiments where bothmembers have a gullet, the distance d, determined from the expressionabove, is apportioned between the corresponding male and female gullets.In one embodiment, the gullet depths of the male and female members areequal to one-half d, but other embodiments use different ratios.

In accordance with the inventive concept, uniform stressing of theclosure threads is achieved in that the thread cross section deflects asa lever in the axial direction and the gullets 15 or 16 facilitate thenecessary deflection properties.

With reference now to the enlargement depicted in FIG. 2 the flanks 17and 17' of gullet 15 are seen to be parallel to each other but this isnot essential since they may also be inclined. Similarly, the flanks l8and 18 of gullet 16 while shown parallel may also be inclined. The radiior curves of the surfaces 19, 20 at the roots of gullets 15, 16, and ofthe surfaces 21, 22 at the roots ofthe threads 13, 14 respectively arepreferably such as to equalize distribution of stress in the vesselsystem so that the threads function as uniformly stressed continuousflexible levers. The flank portions of the threads are seen to bestraight.

FIG. 2a illustrates a conventional buttress thread engagement in whichthe dashed lines represent deflections. The amounts of the deflectionsareexaggerated for purposes of illustration. D1 is the thread pitchline, and 26 shows the parallel mating surfaces in the unstressedcondition. This parallelism is the expected ideal, but it is doubtful ifthis is fully achieved in actual practice under load. Certainly in theloaded or stressed condition, parallelism is to some extent lost, and

the thread tips 24 and 25 could become the loading points, introducingcomplex bending and sheer stresses that are not well understood.

To centralize the stresses along the pitch line and to equalize thelever arms, a crowned thread construction may be used, as shown in FIG.3.

In the modification illustrated in FIG. 3, it will be seen that theflanks 21A of thread 13 are not straight, as in FIG. 2, but rather theflank 21A of thread 13 is crowned at both the tip and root ends asindicated at 23 to assist in accommodating the bending stresses. Ifdesired, similar crowning may be applied to the flanks of thread 14, asshown in FIG. 3a, or the crowning may be applied to thread 14 ratherthan to thread 13 FIG. 3a illustrates a crowned buttress threadengagement which confines the loading to the rocking or osculatingsurfaces 27. It should be noted that thread tips 28 and 29 are unloadedwhich makes for a consistently short lever arm. Crowning permits greatlatitude in the manufacturing operations, since parallel thread surfacesare unnecessary. The dashed line positions of the threads in FIG. 3arepresent, in an exaggerated way,'the deflections to be expected underload conditions. Engagement of the threads under load is shown by thedeflected surfaces, designated 27 What I claim is:

1. In a thread system for use on screw-type closure structures forhigh-pressure vessels and the like wherein a threaded-type plug closuremember is screwed into a complementary threaded opening into the vessel,the improvement wherein a paraboliclike tapered gullet is provided belowthe normal root portion of at least one of the two mating threads, thedepth of the gullet increasing in the direction of the end of the threadnearest the interior of the vessel, the gullet establishing aflexibility characteristic enabling the thread cross section to deflectas a lever in order to enable said closure threads to be uniformlystressed and to accommodate pressurized stress patterns of the vesselsystem.

2. A flexible thread system as defined in claim 1 wherein a gullet isprovided for each of said threads.

3. A flexible thread system as defined in claim 1 wherein the depth ofthe gullet increases in the direction of the end of the thread nearestthe interior of the vessel substantially in accordance with a curveexpressed by the formula:

where:

D thread depth,

B the axial length of the threaded opening,

b the distance along the threads measured from the first load-bearingthread,

d the depth of the gullet measured from the extreme edge of the threadat the distance b.

4. A flexible thread system as defined in claim 1 wherein crowns areprovided at both the tip and root ends of at least one mating thread.

5. A flexible thread system as defined in claim 1 wherein the flanks ofboth of the mating threads are crowned.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,589,5 Dated June 29 1971 Inventor(s) Rossmann It is certified that errorappears in the above-identified patent and that said Letters Patent arehereby corrected as shown below:

Col. 3, line 23, should read 6. 31 X Y 0.75 X Y 0.0156Y l (SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer ActingCommissioner of Patents FORM PO-1050 [IO-69) USCOMM-DC 6037I5-P6Q

1. In a thread system for use on screw-type closure structures forhigh-pressure vessels and the like wherein a threaded-type plug closuremember is screwed into a complementary threaded opening into the vessel,the improvement wherein a paraboliclike tapered gullet is provided belowthe normal root portion of at least one of the two mating threads, thedepth of the gullet increasing in the direction of the end of the threadnearest the interior of the vessel, the gullet establishing aflexibility characteristic enabling the thread cross section to deflectas a lever in order to enable said closure threads to be uniformlystressed and to accommodate pressurized stress patterns of the vesselsystem.
 2. A flexible thread system as defined in claim 1 wherein agullet is provided for each of said threads.
 3. A flexible thread systemas defined in claim 1 wherein the depth of the gullet increases in thedirection of the end of the thread nearest the interior of the vesselsubstantially in accordance with a curve expressed by the formula: 6.31b2d/B2D + 0.75 bd2/BD2 + 0.0156 d3/D3 - 1 0 where: D thread depth, B theaxial length of the threaded opening, b the distance along the threadsmeasured from the first load-bearing thread, d the depth of the gulletmeasured from the extreme edge of the thread at the distance b.
 4. Aflexible thread system as defined in claim 1 wherein crowns are providedat both the tip and root ends of at least one mating thread.
 5. Aflexible thread system as defined in claim 1 wherein the flanks of bothof the mating threads are crowned.